Aqueous electrolytes have emerged as a promising alternative for sodium-ion batteries due to their inherent safety, low cost, and environmental friendliness compared to organic solvent-based systems. The use of water as a solvent eliminates flammability risks while offering high ionic conductivity, which simplifies battery design and manufacturing. However, aqueous systems face significant challenges, including a narrow electrochemical stability window, corrosion of current collectors, and pH instability, which must be addressed to achieve competitive performance.

Salt selection is a critical factor in optimizing aqueous sodium-ion electrolytes. Sodium sulfate (Na2SO4) is widely studied due to its high solubility, low cost, and chemical stability. The neutral pH of Na2SO4 solutions reduces corrosion risks compared to acidic or alkaline alternatives. Sodium perchlorate (NaClO4) offers higher solubility and ionic conductivity but presents safety concerns due to its oxidizing nature. Sodium triflate (NaCF3SO3) has been investigated for its electrochemical stability, though its higher cost limits scalability. The choice of salt influences not only conductivity but also the solvation structure of Na+ ions, which affects electrode kinetics and intercalation behavior.

The electrochemical stability window of aqueous electrolytes is fundamentally limited by water decomposition. Thermodynamically, water undergoes reduction at 0 V vs. SHE (standard hydrogen electrode) and oxidation at 1.23 V, restricting the practical voltage range to approximately 1.5–1.8 V due to overpotentials. This narrow window severely constrains energy density compared to non-aqueous systems. Strategies to expand the stability window include using high-concentration "water-in-salt" electrolytes, where the reduced free water content suppresses hydrogen and oxygen evolution. For example, sodium bis(fluorosulfonyl)imide (NaFSI) solutions at 17 mol/kg have demonstrated stability up to 2.3 V by forming a protective interphase on electrodes.

pH stabilization is essential to prevent parasitic reactions and electrode dissolution. Neutral pH electrolytes (6–8) minimize corrosion but may suffer from carbon dioxide absorption, which forms carbonic acid and lowers pH over time. Buffering agents such as sodium phosphate (Na3PO4/Na2HPO4) or borax (Na2B4O7) help maintain pH stability. Alkaline electrolytes (pH > 10) using NaOH or KOH additives can enhance anode stability but accelerate corrosion of aluminum current collectors. Acidic systems (pH < 4) are generally avoided due to severe hydrogen evolution and cathode dissolution.

Corrosion of current collectors poses a major challenge in aqueous sodium-ion batteries. Aluminum, commonly used in non-aqueous systems, undergoes pitting corrosion in neutral and alkaline media. Stainless steel offers better resistance but increases cost. Carbon-coated metals or titanium substrates have shown improved corrosion resistance but require cost-benefit analysis for large-scale adoption. The corrosion process is exacerbated at high potentials, where oxidation of metal surfaces leads to increased impedance and capacity fade. Additives like sodium molybdate (Na2MoO4) can act as corrosion inhibitors by forming passivation layers.

Solute-solvent interactions in aqueous electrolytes govern Na+ transport and electrode compatibility. In dilute solutions, Na+ ions are fully solvated by water molecules, forming a primary hydration shell with approximately six water molecules per ion. At high concentrations, as in water-in-salt systems, the scarcity of free water changes the solvation structure, reducing water activity and widening the electrochemical window. The viscosity of concentrated electrolytes increases, which may compromise wettability and electrode penetration. Molecular dynamics simulations reveal that anions such as FSI− preferentially adsorb on electrode surfaces, forming a pseudo-solid-electrolyte interphase that inhibits water reduction.

Recent advances in water-in-salt electrolytes have demonstrated improved performance for aqueous sodium-ion batteries. Systems using sodium acetate (NaOAc) at 20 mol/kg have achieved coulombic efficiencies exceeding 99% by suppressing gas evolution. The use of sodium nitrate (NaNO3) in saturated solutions has shown enhanced cathode stability due to nitrate’s oxidative resistance. However, salt precipitation at low temperatures remains a concern for concentrated systems. Hybrid electrolytes combining Na2SO4 with organic co-solvents like ethylene glycol have been explored to lower freezing points without sacrificing stability.

Electrode-electrolyte compatibility is another critical consideration. Prussian blue analogs exhibit excellent stability in aqueous media due to their rigid open framework, which accommodates Na+ insertion with minimal lattice distortion. Manganese-based oxides suffer from Mn dissolution unless electrolyte pH is carefully controlled. Organic electrodes, such as quinone derivatives, demonstrate fast kinetics in aqueous electrolytes but require protection from nucleophilic attack by water molecules.

Future development of aqueous sodium-ion batteries will require balancing multiple factors: expanding the electrochemical window without compromising ionic conductivity, optimizing salt concentration to minimize cost while maximizing stability, and developing corrosion-resistant materials suitable for mass production. Progress in understanding interfacial phenomena and solvation chemistry will be key to overcoming current limitations. The environmental and safety advantages of aqueous systems make them particularly attractive for large-scale energy storage, where cost and longevity outweigh the need for high energy density. Continued research into advanced electrolyte formulations and compatible electrode materials could position aqueous sodium-ion batteries as a viable solution for grid storage and other stationary applications.